USDA lettuce breeders cite germplasm gains

Gains toward achieving resistance to big vein and other costly lettuce diseases have been reported by USDA lettuce geneticists based at Salinas, Calif.

They described their respective projects during the recent annual reports of the California Lettuce Research Board meeting near Coalinga.

The breeders work closely with USDA and University of California plant pathologists and other scientists to develop improved lettuce germplasm through multiple back-crosses and selections of both wild and domesticated plants.

The objective of the board-supported research is to develop disease-resistant features, along with desired horticultural traits, that can be incorporated by seed companies in new commercial varieties.

Big vein, identified by enlarged veins and distorted heads, is blamed on Mirafiori lettuce big vein virus, which researchers use to identify germplasm having greatest potential for development of new resistant cultivars.

According to one of the breeders, Ryan Hayes, big vein resistance has been moved from Lactuca virosa into L. sativa with new hybrids of the two species.

“The important development is we will be using these in germplasm that is independent of Pacific,” said Hayes. Pacific, although a commercial variety having resistance to the disease, has been a problem for some growers.

“We hope to take this research out of the greenhouse breeding for resistance into field trials where we can select for type and find something closer to iceberg or romaine,” Hayes said.

Collaborating in the big vein research is Bill Wintermantel, USDA plant pathologist, who recently developed a new, real-time test for determining the amount of Mirafiori virus and resistance to it in a lettuce sample. The sophisticated test saves much time in the screening process.

Continuing studies on lettuce drop caused by Sclerotinia minor have realigned approaches in selection for resistance by using planting time, bolting, and leaf canopy. Hayes said the program had been using Salinas x PI251246 and other sources, but will be reevaluating new material for slow bolting characteristics and the newly observed trait of “slow dying” associated with the disease.

In efforts to find resistance to Verticillium wilt, Hayes reported that a number of lines have been released for further breeding, and progeny from these are being selected for head type and potential resistance to the wilt’s two races.

Studies in breeding resistance to other diseases and insect pests continue with Hayes’ colleagues, James McCreight, Beiquan Mou, and Ivan Simko.

McCreight said high levels of resistance to lettuce aphid is available in European cultivars from wild lettuce and is being transferred commercially to types for the western U.S.

“Additional sources of resistance are being sought and may prove necessary in the event that the lettuce aphid overcomes the resistance gene,” he said.

In his account of breeding work on Fusarium wilt resistance, McCreight noted that three races of the disease have been found in Japan, although only race I is evident in fields in California and Arizona.

While sources of resistance to the first two are known, none to counter the third race is available. McCreight said a higher resistance than that of Costa Rica No. 4, Salinas, and Salinas 88 is needed for early fall plantings in the desert.

Mou, who’s been working with USDA plant pathologist Carolee Bull on corky root, said selections from Lactuca seriola show promise of conquering a strain of the pathogen found in Watsonville that overcame the known “cor” resistant gene.

Bull, who was on hand to detail her report to the board, said her efforts this year are to aggressively investigate Sphingomonas suberfaciens and related species that could potentially be the reason for the cor gene failure.

Using soil (CA-1) that had been placed in an autoclave to kill soil microorganism, she introduced a strain of the S. suberfaciens pathogen and planted several resistant cultivars, with Salinas as a control, to detect resistance. She found that only the susceptible Salinas variety became infected.

However, she added, all cultivars, including those with resistance, became infected in soil from the Watsonville area where the disease appeared in the fall of 2006.

“It’s very interesting to me that the cultivars that were clearly not susceptible in CA-1 soil are susceptible in the Watsonville soil,” she said. “Something in that soil is causing corky root.”

What’s more, Bull added, she repeated the experiment with different soil samples from the same sources and got similar results.

Bull said she plans to continue the tests and advise interested growers of the results. Those include learning how the pathogen enters the plant and what production practices could encourage the disease.

Part of the difficulty is that as more is learned about it, S. suberfaciens has expanded from one group of strains to several. Other bacteria, which may be the same genus, or perhaps another, appear to also be causing corky root, she said.

Current directions of the corky root breeding studies center on transferring the new source resistance, combined with genes to combat leafminers, downy mildew, and tipburn, which have been released by USDA.

Mou also reported that several transplants from butterhead and wild lettuce crosses made in France show significantly lower leafminer sting densities in a field trial near Salinas.

Ideally, according to Mou, finding a number of genes for resistance to a disease or pest is highly desirable to provide a stronger defense.

Breeder Ivan Simko pointed out that lettuce dieback is caused by a group of soilborne viruses and almost all modern romaine cultivar and most leaf lettuce are susceptible to it.

“To date,” he said, “over 400 modern cultivars, heirlooms, and primitive germplasm were assessed for resistance. Crosses between susceptible and resistant cultivars indicate that a single dominant gene confers resistance in iceberg cultivars.”

Collaborating with scientists at UC, Davis, Simko is evaluating a population of about 230 individuals from a cross between the dieback-susceptible romaine cultivar Valmaine and the resistant iceberg Salinas.